This invention relates to an edge formation process for aluminum solid electrolytic capacitors.
Electrolytic capacitors with excellent high frequency characteristics are in high demand due to speed requirements of circuits for devices such as computers and wireless communications. In addition, high capacitance is required in the low voltage circuits that are used in these devices. Conductive polymers such as polypyrrole, polyaniline, polythiophene, and their derivatives, are finding increasing use as cathodes for electrolytic capacitors because such polymers have much higher conductivity than the liquid electrolytes and manganese dioxide cathodes currently used in these capacitors.
A wet electrolytic capacitor has an anode metal, a dielectric, a liquid electrolyte, and a cathode. Valve metals such as tantalum, aluminum, and niobium are particularly suited for the manufacture of high surface area electrolytic capacitors. The valve metal serves as the anode, and an oxide of the valve metal, coated by electrochemical oxidation of the valve metal surfaces, serves as the dielectric. The process of electrochemically coating a valve metal with a dielectric oxide is called formation. In order to maximize the dielectric surface area, and hence increase the volumetric efficiency of the capacitor, the valve metal substrates are porous bodies. These porous bodies can take the form of etched foils or slugs of compressed powder. The liquid electrolyte is impregnated into the porous body. A high surface area cathode completes the circuit. Etched aluminum foil is a particularly preferred anode material for wet electrolytic capacitors.
In the manufacture of wet aluminum electrolytic capacitors, the aluminum foil is etched to high surface area, coated with a dielectric oxide film, slit to the proper width, and then cut to length. During the slitting and cutting to length operations, the dielectric oxide film on the edges is damaged and bare aluminum is exposed. The foil is then wound, placed in a can (along with the cathode), and filled with a non-aqueous fill electrolyte. The non-aqueous fill electrolyte is composed of, for example, borate in non-aqueous solvents containing a very small amount of water. After filling with electrolyte, the cans are sealed to prevent electrolyte from escaping and to keep additional water out.
A critical part of conditioning a wet aluminum electrolytic capacitor is repairing the damage to edges of the slit, and cut-to-length, foil and any damage to the dielectric oxide on the face of the foil that incurred during the winding operation. If these edges are not re-formed, the capacitor will have a high leakage current. The non-aqueous fill electrolytes, containing a very small amount of water, are very efficient in re-forming oxide on the edges.
In the manufacture of a solid aluminum electrolytic capacitor with a conductive polymer cathode, the foil etching, forming, and slitting, are done in a similar manner to that of wet aluminum electrolytic capacitor. However, the conductive polymer is not efficient at re-forming a dielectric film on the slit and cut edges and at repairing damaged oxide on the face. Therefore, this must be done in a separate step before the conductive polymer is impregnated into the aluminum/aluminum oxide anode.
Re-forming the slit and cut edges can be accomplished by immersing the elements in a formation bath or a series of formation baths. The requirements for these edge formation baths are threefold: 1) They must form a high quality dielectric oxide on the cut edges, 2) They must repair any damage to the dielectric oxide on the face of the element that was damaged during the slitting and cutting to length operation, and 3) They must not damage the dielectric oxide already on the face of the element. In addition, the formed dielectric oxide needs to have excellent hydration resistance.
Hydration resistance is critical for aluminum solid electrolytic capacitors with conductive polymer cathodes. After impregnation with the conductive polymer, the capacitors are washed extensively in water to remove excess reactants and reactant byproducts. This washing is at elevated temperature ( greater than 50xc2x0 C.). The aluminum oxide film is exposed to conditions very conducive to hydration during this washing process, and, therefore, the aluminum oxide film must have a high degree of hydration resistance. Hydration of the oxide during the washing process, or on subsequent storage after washing, can result in hydrated oxide in the weld zone and this hydrated oxide is difficult or impossible to weld through to make a good attachment to the lead frame.
A high degree of hydration resistance is also required during storage or use of capacitors in high humidity environments. If the oxide becomes hydrated during use, the capacitor leakage current will increase, or the capacitor can become a short circuit.
It was discovered that prior art electrolytes have deficiencies when used for edge formation of aluminum anodes intended for used in solid aluminum electrolytic capacitors with conductive polymer cathodes. The fill electrolytes used in wet aluminum capacitors are not suitable for use outside a sealed can because of their toxic nature and their propensity to adsorb water from the air. Thus they cannot be used in open, mass production electrolyte baths.
Electrolytes used for the production of the original aluminum oxide film are also not suitable because they are designed to form oxide on a freshly etched surface or a hydrated oxide surface and not designed to form oxide on cut edges and to repair oxide on the face (cf. U.S. Pat. Nos. 3,796,644; 4,113,579; 4,159,927; 4,481,084; 4,537,665; 4,715,936).
Slitting and cutting the foil to length mechanically damages the edges and this mechanical damage should be repaired before or during the formation of the dielectric oxide film on the edge.
Acids such as oxalic and sulfuric acid produce a thick, porous, non-dielectric oxide films on aluminum. The process of coating a thick, porous, non-dielectric oxide on aluminum is called anodization. The use of these acids in aluminum anodization in combination with a further formation of a high quality dielectric oxide in salts of boric acid are known for high voltage aluminum oxide films (Dekker and van Geel, 1947, Dekker and Middelhoek, 1970, U.S. Pat. No. 5,078,845). Since these acids dissolve some aluminum and aluminum oxide they can also be used to smooth the edge of the slit and cut foil so that the edge is strengthened and a quality barrier layer can be formed beneath them on the edge. EP 1,028,441A1 teaches the use of oxalic acid in combination with ammonium adipate to repair mechanical damage to the edge, coat a thick, porous base layer (both via oxalic acid anodization), and finally coat a dielectric layer beneath the porous layer (via ammonium adipate formation). However, under conditions at which acids such as oxalic and sulfuric are able to produce this thick, porous layer, they are extremely aggressive to the aluminum oxide film already formed on the face of the foil. Thus, the quality properties, such as hydration resistance, of the preexisting aluminum oxide are impaired. Moreover, ammonium adipate is incapable of restoring hydration resistance to the dielectric oxide on the face of the foil or forming a hydration-resistant oxide on the edges of the foil. The combination of oxalic acid and ammonium adipate is also incapable of forming a hydration-resistant oxide on the edges of the foil.
Several electrolyte systems have been considered for the edge formation of aluminum electrolytic capacitors with a solid conductive polymer cathode that overcomes the deficiencies of the prior art. Use of aqueous solutions of ammonium citrate and ammonium dihydrogen phosphate singly or in combination results in a hydration resistant oxide on the edge, but the initial leakage current is higher and capacitance is lower than when oxalic acid is used in combination with aqueous ammonium adipate. However, the oxalic acid anodization followed by formation in dicarboxylic acid salts, such as ammonium adipate, leads to an oxide with no hydration resistance, and thus the parts are unstable towards welding and long-term performance. Thus, a edge formation system that combines the high capacitance and low leakage current performance of the oxalic acid/ammonium adipate system with the hydration resistance of the ammonium citrate or ammonium phosphate systems is desired.
It was discovered that edge formation in an aqueous oxalic acid solution, followed by formation in an aqueous citrate solution, followed by formation in an aqueous phosphate solution imparts high hydration resistance to foil, and when the foil is impregnated with a conductive polymer and fabricated into a capacitor, high capacitance and low leakage current also are obtained.
The invention is directed to a process for edge forming a slit and cut-to-length foil having a dielectric oxide film on at least one surface comprising anodizing the foil in an aqueous oxalic acid electrolyte, further forming the foil in an aqueous citrate electrolyte, preferably dibasic ammonium citrate electrolyte, depolarizing the foil, and then forming the foil in an aqueous phosphate electrolyte, preferably an ammonium dihydrogen phosphate.
The invention is further directed to a solid electrolytic capacitor comprising a foil coated with a dielectric oxide film, wherein the coated foil has slit or cut edges, and the slit or cut edges have been reformed by anodizing the foil in an aqueous oxalic acid electrolyte, then forming the foil in an aqueous citrate electrolyte, then depolarizing the foil, and then forming the foil in an aqueous phosphate electrolyte.